1. Field of the Invention
The present invention relates to an FMX stereophonic receiver and, more particularly, to an FMX stereophonic receiver able to prevent deterioration of stereophonic channel separation caused by field strength variation.
2. Description of the Prior Art
FMX stereophonic broadcasting has been proposed as one means of enlarging the service area and improving the signal-to-noise ratio characteristics of FM stereo broadcasts. The transmission signal of the aforementioned FMX stereo broadcast includes a compressed stereo difference signal (L-R)' broadcast simultaneously with the transmission signal of conventional FM stereo broadcasting, for example, a stereo sum signal (L+R) and stereo difference signal (L-R). The transmission signal can be expressed as: EQU f(t)=(L+R)+Psin(w/2)t+(L-R)sinwt+(L-R)'coswt (1)
where L+R is a stereo sum signal, L-R is a stereo difference signal, P is a stereo pilot signal, and w is the subcarrier angular frequency. As shown by aforementioned Equation (1), compressed stereo difference signal (L-R)' is quadrature modulated from uncompressed stereo difference signal (L-R), resulting in an FMX stereo broadcast transmission signal spectrum shown in FIG. 1.
Furthermore, the relationship between the uncompressed stereo difference signal (L-R) and the compressed stereo difference signal (L-R)' is as shown in FIG. 2 which expresses the compression characteristics. In FIG. 2, when the input signal level is low, the aforementioned signal (L-R)' is 20 dB greater than the uncompressed stereo difference signal (L-R) and, at the same time, input/output characteristics become linear, and also the compression ratio becomes 1:1. When the level of the input signal is medium (approximately -30 dB), the compression ratio becomes .infin.:1, and input/output characteristics are flat over a range of approximately 10 dB. When the input signal level becomes high, the aforementioned signal (L-R)' rapidly attenuates. Therefore, compressed stereo difference signal (L-R)' is as shown by solid line B in FIG. 2 with respect to stereo difference signal (L-R) (solid line A), and the sum signal of the aforementioned signal (L-R) and the aforementioned signal (L-R)' is as shown by dotted line C in FIG. 2.
As discussed above, the transmission signal for FMX stereophonic broadcasting is received by a receiver as shown in FIG. 3. In FIG. 3, the FMX stereophonic broadcast transmission signal received by antenna 1 is received by a receiving circuit 2 of the same construction as a conventional FM stereophonic receiver in which stereo sum signal (L+R) (hereafter referred to as M), stereo difference signal (L-R) (hereafter referred to as S), and compressed stereo difference signal (L-R)' (hereafter referred to as S') are each demodulated. When the received signal is detected by the FM detection circuit included in the receiving circuit, stereo sum signal M is demodulated. When the stereo composite signal is detected by the synchronous detection using the 38-kHz subcarrier signal obtained from the PLL in the receiving circuit, uncompressed stereo difference signal S is demodulated. And when the stereo composite signal is detected by the quadrature detection, compressed stereo difference signal S' is demodulated.
Uncompressed and compressed stereo difference signals S and S' obtained from receiving circuit 2 are added by adder 3, and the result is applied to VCA (voltage control amplifier) 4 operating as an attenuator. When stereo difference signal S and output signal (S+S') of VCA 4 are greater than a specified level (a knee-point level), first and second level detection circuits 5 and 6, each having a threshold level, operate in such a manner that the level of stereo difference signal S and the level of aforementioned output signal (S+S') of VCA 4 are respectively detected by first and second level detection circuits 5 and 6, and are compared by comparator circuit 7. Next, a signal according to the level difference obtained from aforementioned comparator circuit 7 is rectified and smoothed by rectifying circuit 8, and the rectified signal is applied to VCA 4 as a control signal. The output signal (S+S') of aforementioned VCA 4 is controlled by this control signal to be equal to the level of stereo difference signal S. However, when aforementioned stereo difference signal S and output signal (S+S') of VCA 4 are below the knee-point level, first and second level detection circuits 5 and 6 do not operate, and attenuation at VCA 4 is fixed at approximately 20 dB.
Although stereo sum signal M obtained from receiving circuit 2 is applied directly to matrix circuit 9, stereo difference signal S or output signal (S+S') of VCA 4 are selected by switch 10, and applied to matrix circuit 9. Although not given in the above description, a 10-Hz ID signal is included in the FMX stereophonic broadcast transmission signal, and FMX stereophonic broadcasts are differentiated from conventional FM stereophonic broadcasts by the aforementioned ID signal. In addition, because a detection circuit which detects the aforementioned ID signal is built in to receiving circuit 2, whether the broadcast is FMX stereo or not can be determined with the output signal of the aforementioned detection circuit. Switch 10 is controlled by the aforementioned ID signal. When the ID signal is present, switch 10 is switched to a position as shown in FIG. 3. Accordingly, stereo sum signal M and output signal (S+S') from level controlled VCA 4 are matrixed, and left and right stereo signals L and R are generated at left and right output terminals 11 and 12. Furthermore, when the ID signal is not present, switch 10 is switched to a position opposite to that shown in FIG. 3, and stereo sum signal M and stereo difference signal S are matrixed in matrix circuit 9.
As described above, because FMX stereophonic broadcast system uses compressed and expanded stereo difference signal S, it is possible to achieve significant improvements in the S/N ratio, and the service area can be enlarged comparably equal to that of the conventional monaural FM broadcast system.
It is to be noted that the FMX stereophonic broadcast transmission signal can be accurately received by a conventional FM stereophonic receiver. In this case, compressed stereo difference signal S' is quadrature modulated with respect to stereo difference signal S, and reception is not adversely affected.
Details concerning FMX stereophonic broadcasting are disclosed, for example, in an article "Improving the Signal-to-Noise Ratio and Coverage of FM Stereophonic Broadcasts" by Emil L. Torick and Thomas B. Keller in "JOURNAL OF THE RADIO ENGINEERING SOCIETY", volume 33, number 12, issued December 1985.
A conventional FM stereophonic receiver is equipped with a separation control circuit to improve the signal-to-noise ratio during the receiving of signals having weak field strength. When the field strength of the receiving FM stereophonic signal drops below a predetermined level, the receiver is switched automatically and continuously from a stereophonic mode to a monaural mode by a stereophonic separation control circuit, because if the stereophonic mode were to be still maintained, there will be unpleasant noise signals in the stereophonic sounds. In other words, there is a lower limit of the field strength of the FM stereophonic signal at which the stereophonic mode is available (hereinafter referred to as a stereophonic mode holding lower limit). It has been found that the FM stereophonic mode holding lower limit is higher than the FMX stereophonic mode holding lower limit. For instance, according to one example, the FM stereophonic mode holding lower limit is about 40 dB, whereas the FMX stereophonic mode holding lower limit is about 20 dB.
This can be explained as follows. In the case of an FMX stereophonic broadcast system, the signal-to-noise ratio is improved through expansion and compression, such that the signal-to-noise ratio is approximately equal to that of monaural broadcast receiving. As a result, if a separation control circuit operates similarly to FM stereophonic broadcast receiving during FMX stereophonic broadcast receiving, deterioration of channel separation will become a greater problem than improvement of the signal-to-noise ratio. Nevertheless, it has been confirmed that if there is an extreme drop in the field strength of the receiving signal during FMX stereophonic broadcast receiving, there will be a corresponding severe drop in the signal-to-noise ratio.